update broken images

Author: RCmags

collections/projects/Airplanes/_posts/2019-09-19-stabilized_airplane.md

@@ -1,6 +1,6 @@
 ---
 layout: post
-title: "Tail-less airplane stabilized by an angle of attack sensor"
+title: "Review | Tail-less airplane stabilized by an angle of attack sensor"
 
 # image sliders:
 slider1:

collections/projects/Helicopters/_posts/2017-07-01-heli_flight_controller.md

@@ -5,8 +5,7 @@ title: "Review | Flight controller for a weight-shift RC helicopter"
 
 # Introduction to concept
 
-Helicopters are naturally unstable aircraft. Without a system to augment their stability they will inevitably spiral towards the ground. Thankfully, theres a few ways to enhance their stability. A classic approach is to use a heavy bar placed at an angle relative to the rotor blades. Such a 
-system is known as a flybar, stabilizer-bar, or mechanical gyro.  
+Helicopters are naturally unstable aircraft. Without a system to augment their stability they will inevitably spiral towards the ground. Thankfully, theres a few ways to enhance their stability. A classic approach is to use a heavy bar placed at an angle relative to the rotor blades. Such a system is known as a flybar, stabilizer-bar, or mechanical gyro.  
 
 ![image](/img/heli_controller/HillerHead500pics.gif)
 [See: Bell-Hiller mixing]()
@@ -33,9 +32,9 @@ This project concerns itself with the development of a simple and lightweight fl
 
 The pitch and roll axes of the helicopter can be stabilized by means of two [PID Controllers](https://en.wikipedia.org/wiki/PID_controller). The angular velocity, as measured by a gyroscope, acts as the proportional term of the control loops. By taking time integral of this value, one obtains the angular deflection of the aircraft. Hence, the proportional term will act like a viscous damper, while the integral term will act like a spring. 
 The derivative term would represent the angular acceleration of the aircraft, although in practice may be used to smoothen in the proportional term. 
-  
-![image](https://cdn.instrumentationtools.com/wp-content/uploads/2018/01/PID-Controller-Responses-to-Two-momentary-step-inputs.png)
-  
+
+![image](/img/heli_controller/pid-response.jpg)
+
 The yaw axis is different as it is affected by the torque generated by the rotor, and this in turn depends on the configuration of the helicopter. For an aircraft with a tail rotor, the yaw axis can use a PID controller along with a counter-torque term that increases with throttle. This is known as [revo mixing](https://helihack.co.uk/revomixing).  
 
 These corrections can improve the short-term stability of a helicopter, but their effect on long-term stability is less clear. Nonetheless, it is not critical that the aircraft finds an upright attitude on its own. It can still be unstable so long it occurs slowly. This way the pilot can provide nessesary corrections and the helicopter can remain aloft.